Unit Testing Basics

Learn how to write and run unit tests for C# code using xUnit, one of the most popular .NET testing frameworks.

Why Unit Test?

A unit test verifies that a small piece of code (usually a single method) behaves correctly, automatically and repeatably — catching regressions before they reach production.

Setting Up xUnit

xUnit is one of the most popular .NET testing frameworks. Create a dedicated test project alongside your main project:

dotnet new xunit -o MyApp.Tests
cd MyApp.Tests
dotnet add reference ../MyApp/MyApp.csproj

Writing a Test

public class Calculator
{
    public int Add(int a, int b) => a + b;
}

public class CalculatorTests
{
    [Fact]
    public void Add_ReturnsSumOfTwoNumbers()
    {
        var calculator = new Calculator();

        int result = calculator.Add(2, 3);

        Assert.Equal(5, result);
    }
}

Running tests

dotnet test

Example Output

Click Run to see what this code prints.

Assertions

Method	Checks
Assert.Equal(expected, actual)	Two values are equal
Assert.True(condition)	A condition is true
Assert.Throws<T>(action)	An action throws a specific exception
Assert.Null(value)	A value is null

Theory for Multiple Cases

Use `[Theory]` with `[InlineData(...)]` to run the same test logic against multiple sets of inputs, instead of copy-pasting near-identical [Fact] tests.

FAQs

Is xUnit the only testing framework for .NET?

No — NUnit and MSTest are also widely used. xUnit is currently the most common choice for new ASP.NET Core projects.

Summary

Unit tests verify small pieces of code automatically and repeatably.
[Fact] tests a single case; [Theory] with [InlineData] tests multiple cases.
Assert methods check expected outcomes and fail the test if they don't hold.

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